In recent years, various designs and functions of shoes are demanded according to the various needs of consumers. Even in the sport industry as well as the outdoor industry, there have been proposed shoes having a light weight as well as a distinctive function.
As a method of lightening a shoe, a midsole may be formed by injection-foaming an EVA material such that the weight of the midsole is lightened.
According to a method of manufacturing a midsole by injection foam molding, after a liquid raw material containing a foaming material is injected into a mold having a size slightly smaller than that of the midsole to be manufactured, heat and pressure are applied to the mold, such that chemical reaction is made inside the mold, thereby performing molding in a shape of a midsole. By opening the mold after a predetermined time period, the molded article in the mold is instantaneously expanded at the same time as the mold is opened, so that a midsole having a desired size is produced. Such a method of manufacturing a midsole through the injection foaming method has been well known in the art and thus, the details will be omitted.
Meanwhile, at present, to pierce the side of a midsole, a midsole which are separated into two pieces of which each or one has a groove formed in a predetermined shape or depth is prepared and then, the pieces of the sole are assembled by means of adhesive or the like, thereby manufacturing a side pierced midsole.
However, according to such a method, since the midsole is implemented by bonding, it is difficult to not only manufacture the midsole but also ensure the quality of the product, and the effect of buffering by the through-hole formed through the side of the midsole is deteriorated.
Therefore, to solve the problem described above, the applicant of the present application had proposed a midsole in which a through-hole is integrally formed at a side thereof, so that the midsole has excellent buffering function and the manufacturing difficulty caused due to adhesion is overcome. (Korean Unexamined Patent Publication No. 10-2013-0027610 A).
However, in the injection foam molding method described above, it is very difficult to smoothly form a through-hole perforated in the lateral direction. The reason is that a shaft mold is required to penetrate through the center of a mold to integrally form the through-hole perforated in the lateral direction in the midsole, and an error occurs in the structure in which the shaft metal mold is closely attached to the inner wall of the mold so that it is difficult to manufacture a complete product.
That is, in order to form the through-hole in the lateral direction of the midsole, the shaft mold for forming the through-hole in the midsole is installed inside the mold for molding the midsole. In the case where the midsole is molded at a constant temperature and pressure, for example, by a foaming method in the state that the shaft mold is installed inside the mold and is not tightly closed to the mold, a gap is generated between the mold and the shaft mold installed inside the mold and the molded material enters the gap, so that the through-holes of the midsole are partially or fully blocked. Thus, a separate processing means is required to remove such a material formed in the gap.
That is, although the injection molding method is characterized in that molding is performed at a very high pressure, when the shaft mold is simply placed to be tightly closed to the inner wall surface of the upper and lower molds, since a gap is formed between the shaft and the inner wall of the mold, the result of molding is not good. Even if a mold is manufactured by precision machining in the production of a mold, a bur is generated in the result of molding.
Therefore, after the molding is completed, the post-processing is necessarily required, so that the mold manufacturing cost is excessive and the production cost of the outsole is increased due to excessive labor as well as excessive working equipment and working time.
In addition, to form the through-hole perforated in the lateral direction of the midsole, the mold opening structure is formed in the same line as the shaft mold installed inside the mold, so that the parting line is exposed on the inner wall of the through-hole of the molded midsole.
Meanwhile, when the midsole is individually molded by injection foam molding, a defect may occur in the midsole. That is, after rapid foaming by the opening of the mold in the production process, the volume is reduced due to the temperature change caused by making contact with external air having a relatively low temperature while being cooled, thereby causing a dimensional defect.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.